In recent years, in association with advance in the image processing technology and the image forming technology, images are so exquisitely and accurately formed that sometimes, for instance, a copied bill copied by a color copying machine can not be distinguished from a real one. For this reason, now efforts are made for development of an apparatus which can detect a copy-inhibited document such as bills or marketable securities which are inhibited to be copied and inhibits illegally copying copy-inhibited documents.
As a method of detecting copy-inhibited documents such as bills or marketable securities, there have been disclosed, for instance, a method disclosed in Japanese Patent Laid Open Publication No.83571/1990 disclosing an "image recording apparatus" in which input image data is compared to a particular mark (pattern data) previously registered therein by way of the pattern matching method and which determines that a document is a copy-inhibited one when the particular mark exists in the input data, and a method disclosed in Japanese Patent Laid Open Publication No.54681/1992 disclosing a "color image processing apparatus" which detects a copy-inhibited document by comparing a form of histogram (color tome data) based on color phase distribution.
In case of Japanese Patent Laid Open Publication No.54681/1992, however, although disturbance of input image data is corrected by executing such processing as smoothing to the image data to improve accuracy in detection, the determination is executed by comparing a form of histogram based on color distribution, so that, if the tone of image data is intentionally changed, accuracy in detecting copy-inhibited documents becomes lower and sometimes a copy-inhibited document may be mistaken for a general document.
For this reason, in a conventional type of apparatus detecting a copy-inhibited document using color tone data, there is the possibility that a copy-inhibited document copied by intentionally changing color tone of image data and also changing tone of a color of toner or ink or the like used in an image output device such as a printer may be overlooked.